This invention relates to magnetometers and more particularly to a Helium free precession magnetometer.
A .sup.3 He free precession magnetometer operates in three stages. In the first stage, the pumping stage, a glass cell containing low pressure .sup.3 He gas (1 to 10 Torr) is excited with an electric discharge and radiation from a .sup.4 He lamp is directed through the cell. The interaction of the .sup.4 He light and the .sup.3 He atoms tends to magnetize the .sup.3 He atoms in a direction parallel to the direction of the ambient magnetic field. In the second stage, the nuclear magnetization of the cell is rotated and in the third stage, the cell magnetization freely precesses for several hours.
In the past, rotation of the nuclear magnetization of the cell of .sup.3 He gas was accomplished, after the establishment of the cell magnetization along the direction of the ambient field (H.sub.o), by applying a magnetic bias field H.sub.B to the cell in opposition to the earth's field. The field is applied slowly so that the magnetization can follow the resultant field adiabatically to a direction making an angle with the field (H.sub.o). Then the bias field is rapidly (non-adiabatically) removed so that the magnetization remains instantaneously in position and begins to precess around the field (H.sub.o). A more detailed discussion of this technique is set forth in an article by R. E. Slocum and B. I. Marton, "A Nuclear Free Precession Magnetometer Using Optically Polarized .sup.3 He Gas", IEEE Trans. Magnetics, MAG-10 (1974).
Several problems have been found to exist with the prior art devices. First, a bias field strength of the order of one Gauss is required. In order to produce a one Gauss field of sufficient uniformity, a coil with dimensions may times larger than the cell dimensions is necessary as well as some knowledge about the earth's field direction in order to make the device work. Secondly, circuitry of some complexity is required to extinguish the bias field quickly.